Exhaust emission control device of internal combustion engine

ABSTRACT

The present invention is provided with an NOx catalyst  17  provided in the exhaust passage of an internal combustion engine capable of lean burn and adapted to release and reduce absorbed NOx by a reducing agent, a reducing agent supplying means  19  provided in the exhaust passage on the upstream side of the NOx catalyst  17,  a load detecting means for detecting the load of an internal combustion engine  1,  and a reducing agent addition control means for controlling the reducing agent addition period and the reducing agent addition interval on the basis of the load of the internal combustion engine  1.  The addition period and the addition interval for the reducing agent are controlled taking into account, for example, the degree to which reducing agent adheres to the wall surface in the exhaust passage depending on the velocity, temperature, etc., of the exhaust of the internal combustion engine  1,  whereby a sufficient amount of reducing agent is supplied to the NOx catalyst side independently of the operating condition. Thus, even if the operating condition of the internal combustion engine is changed, it is always possible to supply an appropriate amount of reducing agent to the NOx catalyst.

TECHNICAL FIELD

[0001] The present invention relates to an exhaust emission controldevice for cleaning exhaust gas discharged from an internal combustionengine capable of lean burn.

BACKGROUND ART

[0002] Examples of an exhaust emission control device for cleaningexhaust gas discharged from an internal combustion engine capable oflean burn, such as a diesel engine or a lean burn gasoline engine,include NOx catalysts, such as a selective reduction type NOx catalystand an occlusion reduction type NOx catalyst.

[0003] The selective reduction type NOx catalyst is a catalyst whichreduces or decomposes NOx in an atmosphere with excess oxygen in thepresence of hydrocarbon (HC). To clean NOx with the selective reductiontype NOx catalyst, an appropriate amount of HC component (hereinafterreferred to as reducing agent) is required.

[0004] When using the selective reduction type NOx catalyst for theexhaust emission control of an internal combustion engine as mentionedabove, the amount of HC component in the exhaust gas during normaloperation of the internal combustion engine is very small, so that inorder to clean NOx during normal operation, it is necessary to supply asa reducing agent, for example, light oil serving as fuel to theselective reduction type NOx catalyst with.

[0005] On the other hand, the occlusion reduction type NOx catalystabsorbs NOx when the air fuel ratio of the inflow exhaust gas is lean,and releases the absorbed NOx when the oxygen concentration of theinflow exhaust gas is reduced, effecting reduction to N₂.

[0006] When using the occlusion reduction type NOx catalyst for exhaustemission control of the internal combustion engine, the NOx in theexhaust gas will be absorbed by the NOx catalyst since the air fuelratio of the exhaust gas during normal operation of the internalcombustion engine is lean. However, when exhaust gas of lean air fuelratio is continued to be supplied to the NOx catalyst, the NOx absorbingcapacity of the NOx catalyst attains saturation, and no further NOx canbe absorbed, with the result that the NOx in the exhaust gas is allowedto leak.

[0007] In view of the above, in the occlusion reduction type NOxcatalyst, it is necessary to reduce the oxygen concentration by makingthe air fuel ratio of the inflow exhaust gas rich with a predeterminedtiming before the NOx absorbing capacity attains saturation, and torelease the NOx absorbed by the NOx catalyst to reduce it to N₂, therebyrecovering the NOx absorbing capacity of the NOx catalyst. In thefollowing, this operation of temporarily making the air fuel ratio ofthe inflow exhaust gas rich will be referred to as rich spike.

[0008] On the other hand, to recover the NOx absorbing capacity of theNOx catalyst, it is necessary to appropriately enrich the air fuel ratioof exhaust. Conventionally, the pattern in which fuel as reducing agentis added and the target fuel addition pressure are set on the basis of amap indicating the relationship between the internal combustion engineRPM and fuel injection amount obtained through experiment.

[0009] However, when supplying reducing agent at the exhaust port of aninternal combustion engine, the exhaust port and the NOx catalyst areusually spaced apart from each other, and the reducing agent is noteasily carried by the exhaust flow depending upon the operatingcondition of the internal combustion engine, resulting in a rather poorefficiency in the supply of reducing agent. Specifically, in the rangewhere the internal combustion engine operates at low speed and withsmall load, the exhaust velocity is low, and the exhaust gas temperatureis low, so that a part of the reducing agent added adheres to the wallsurface in the exhaust passage, with the result that the degree of richspike at the exhaust port differs from the degree of rich spike at theNOx catalyst. In other words, even if the rich spike on the exhaust portside attains the target air fuel ratio, the amount of reducing agentreaching the NOx catalyst decreases due to the adhesion to the wallsurface on the NOx catalyst side, resulting in a decrease in the degreeof richness. Further, the rich spike, effected instantaneously, involvesa delay in reaction on the NOx catalyst side, with the result that therich period is extended and the degree of richness becomes lower thanthe target air fuel ratio.

[0010] For example, even if the degree of rich spike at the exhaust portis approximately the one as shown in FIG. 6(A), the degree of richnessis lowered with the NOx catalyst as shown in FIG. 6(B). Thus, the targetair fuel ratio cannot be attained with the NOx catalyst, and the releaseand reduction of NOx cannot be effected to a sufficient degree. Then, insome cases, the NOx absorbing capacity of the NOx catalyst attainssaturation and cannot be recovered, resulting in leakage of NOx in theexhaust.

[0011] The present invention has been made in view of the aboveproblems. It is an object of the present invention to provide an exhaustemission control device for an internal combustion engine capable ofsupplying an appropriate amount of reducing agent to NOx catalyst evenif the operating condition of the internal combustion engine changes.

DISCLOSURE OF THE INVENTION

[0012] The present invention relates to an exhaust emission controldevice for an internal combustion engine, characterized by comprising:

[0013] an NOx catalyst provided in an exhaust passage of an internalcombustion engine capable of lean burn and adapted to release absorbedNOx by a reducing agent for reduction;

[0014] a reducing agent supplying means provided in the exhaust passageon the upstream side of the NOx catalyst;

[0015] a load detecting means for detecting the load of the internalcombustion engine; and

[0016] a reducing agent addition control means for controlling reducingagent addition period and reducing agent addition interval on the basisof the load of the internal combustion engine.

[0017] In accordance with the present invention, it is desirable tofurther provide an addition determining means for determining as towhether reducing agent is to be added or not according to the operatingcondition of the vehicle, the addition amount of reducing agent and thereducing agent addition interval being controlled on the basis of theload detected by the load detecting means when it is determined by theaddition determining means that addition is possible.

[0018] It is desirable that a judgment be made by the additiondetermining means as to whether the NOx catalyst is at an activationtemperature or not, whether the operation range of the internalcombustion engine is within the range where addition of reducing agentis possible or not, and the like, reducing agent being supplied onlywhen release and reduction of NOx is possible, thereby preventing thereducing agent from passing through the NOx catalyst.

[0019] Further, when the load of the internal combustion engine issmall, it is possible to increase the amount of reducing agent added andelongate the reducing agent addition interval as compared to the case inwhich the load of the internal combustion engine is large.

[0020] Further, the present invention relates to an exhaust emissioncontrol device for an internal combustion engine, characterized bycomprising:

[0021] an NOx catalyst provided in an exhaust passage of an internalcombustion engine capable of lean burn and adapted to release absorbedNOx by a reducing agent for reduction;

[0022] a reducing agent supplying means provided in the exhaust passageon the upstream side of the NOx catalyst;

[0023] an operating condition detecting means for detecting the load andan RPM of the internal combustion engine; and

[0024] a reducing agent addition control means for controlling reducingagent addition period and reducing agent addition interval on the basisof the detected load and the RPM of the internal combustion engine.

[0025] In the present invention, when the load and the RPM of theinternal combustion engine are low, the velocity of the exhaust is low,and the reducing agent is not easily carried by the exhaust flow, sothat the addition period and addition interval for the reducing agentadded are controlled according to the engine load and the engine RPM.That is, when the load and RPM of the internal combustion engine arelow, the addition period and addition interval for the reducing agentare increased. On the other hand, as the engine load increases, theaddition period and addition interval for the reducing agent arereduced. The addition period and addition amount for the fuel are in afixed relationship unless the addition pressure (fuel pressure) of thereducing agent varies, so that the addition amount of the reducing agentis in proportion to the addition period thereof.

[0026] Thus, in accordance with the present invention, the additionperiod and the addition interval for the reducing agent are controlledwhile taking into account, for example, the degree to which reducingagent adheres to the wall surface in the exhaust passage depending onthe velocity, temperature, etc. of the exhaust of the internalcombustion engine, making it possible to always supply a sufficientamount of reducing agent to the NOx catalyst side independently of theoperating condition.

[0027] Further, when the load and RPM of the internal combustion engineare low, the amount of NOx generated is small, so that, by elongatingthe addition interval, the reducing agent previously added interfereswith that subsequently added in the condition in which the velocity ofthe exhaust is low, whereby it is possible to prevent the degree ofrichness from increasing excessively.

[0028] Regarding the exhaust emission control device of the presentinvention, examples of an internal combustion engine capable of leanburn may include an in-cylinder direct injection type lean burn gasolineengine and a diesel engine.

[0029] The detection of the load by the operating condition detectingmeans can be effected on the basis, for example, of an output signal ofan accelerator opening sensor or an output signal of an airflow meterindicating intake air amount. The detection of the engine RPM can beeffected through calculation of an output pulse, for example, of a crankangle sensor.

[0030] Examples of the NOx catalyst in the exhaust emission controldevice of the present invention may include an occlusion reduction typeNOx catalyst and a selective reduction type NOx catalyst.

[0031] The occlusion reduction type NOx catalyst is a catalyst whichabsorbs NOx when the air fuel ratio of inflow exhaust gas is lean andreleases the absorbed NOx for reduction to N₂ when the oxygenconcentration of the inflow exhaust gas decreases. The occlusionreduction type NOx catalyst uses, for example, alumina, as the support,which supports thereon, for example, at least one selected from thegroup consisting of: an alkali metal, such as potassium K, sodium Na,lithium Li, or cesium Cs, an alkaline earth metal, such as barium Ba, orcalcium Ca, and a rare earth metal, such as lanthanum La or yttrium Y,and a precious metal, such as platinum Pt.

[0032] The selective reduction type NOx catalyst is a catalyst whichreduces or decomposes NOx in an excess oxygen atmosphere and in thepresence of hydrocarbon and it includes a catalyst in which zeolitesupports a transition metal, such as Cu, that has undergone ionexchange, a catalyst in which zeolite or alumina supports a preciousmetal, and the like.

[0033] In the exhaust emission control device of the present invention,the reducing agent adding means may be formed by a reducing agent supplypump, a reducing agent injection nozzle provided in the exhaust passage,etc.

[0034] In the exhaust emission control device for an internal combustionengine of the present invention, even if the reducing agent addingposition and the NOx catalyst are spaced apart from each other, thereducing agent can be supplied in an appropriate manner by controllingthe addition period and the addition interval for the reducing agent. Inparticular, when the RPM and load of the internal combustion engine arelow, the addition period and addition interval for the reducing agentare elongated, whereby, even if a part of the reducing agent adheres tothe wall surface in the exhaust passage, it is possible for a sufficientamount of reducing agent to be carried to the NOx catalyst by theexhaust flow.

[0035] In this way, independently of the operating condition of theinternal combustion engine, it is possible to cause an appropriateamount of reducing agent to reach the NOx catalyst, thereby effectingNOx reduction efficiently. Thus, it is actually possible to constantlymaintain the NOx occlusion amount by the NOx catalyst at a level nearzero, thereby achieving a high level of efficiency in NOx cleaning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic diagram showing an exhaust emission controldevice for an internal combustion engine according to the presentinvention;

[0037]FIG. 2 is a diagram illustrating the absorption and releaseoperations of NOx by an occlusion reduction type NOx catalyst;

[0038]FIG. 3 is a diagram showing the relationship between fuel additioninjection period, engine RPM, and fuel injection amount;

[0039]FIG. 4 is a diagram showing the relationship between fuel additioninterval, engine RPM, and fuel injection amount;

[0040]FIG. 5 is a flowchart illustrating a fuel addition routine; and

[0041]FIG. 6 is a diagram showing the rich spike air fuel ratios at theexhaust port and the NOx catalyst.

BEST MODE FOR CARRYING OUT THE INVENTION

[0042] The exhaust emission control device for an internal combustionengine in accordance with an embodiment of the present invention willnow be described with reference to FIGS. 1 through 6. Note that, in thefollowing embodiment, the exhaust emission control device for aninternal combustion engine of the present invention is applied to adiesel engine for driving a vehicle.

[0043]FIG. 1 is a diagram showing the general construction of an exhaustemission control device for an internal combustion engine according tothis embodiment. In the drawing, an engine 1 is a straight four-cylinderdiesel engine, and intake air is introduced into the combustion chamberof each cylinder through an intake manifold 2 and an intake pipe 3. Atthe start end of the intake pipe 3, there is provided an air cleaner 4,and, in the midway of the intake pipe 3, there are provided an airflowmeter 5, a compressor 6 a of a turbo charger 6, an intercooler 7, and athrottle valve 8.

[0044] The airflow meter 5 outputs an output signal in correspondencewith the amount of new air flowing into the intake pipe 3 through theair cleaner 4 to an engine control electronic control unit (ECU) 9,which calculates the intake air amount on the basis of the output signalof the airflow meter 5.

[0045] Further, fuel (gas oil) is injected into the combustion chamberof each cylinder of the engine 1 from a fuel injection valve 10. Eachfuel injection valve 10 is connected to a common rail 11, to which fuelis supplied from a fuel pump 12. The fuel pump 12 is driven by a crankshaft (not shown) of the engine 1. The opening time and opening periodof each fuel injection valve 10 are controlled by the ECU 9 according tothe operating condition of the engine 1.

[0046] The exhaust gas generated in the combustion chamber of eachcylinder of the engine 1 is discharged to an exhaust pipe 16 through anexhaust manifold 14, and discharged to the atmosphere through a muffler(not shown). A part of the exhaust gas discharged to the exhaustmanifold 14 can be recycled to the intake manifold 2 through an exhaustreflux pipe 23, and an EGR cooler 24 and an EGR valve 25 are provided inthe midway of the exhaust reflux pipe 23. The opening of the EGR valve25 is controlled by the ECU 9 according to the operating condition ofthe engine 1, controlling the exhaust reflux amount.

[0047] Provided in the midway of the exhaust pipe 16 are a turbine 6 bof a turbo charger 6 and a casing 18 accommodating an occlusionreduction type NOx catalyst (lean NOx catalyst). The turbine 6 b isdriven by the exhaust gas, which drives a compressor 6a connected to theturbine 6 b to raise the pressure of the intake air.

[0048] Further, a fuel addition nozzle (addition port of a reducingagent adding device) 19 is mounted to the cylinder head 30 of the engine1 so as to face the exhaust port 13 of the fourth cylinder. To the fueladdition nozzle 19, fuel pumped up by the fuel pump 12 can be suppliedthrough a fuel pipe 20 and a fuel passage 21 provided in the cylinderhead 30, the addition amount being controlled by a control valve 22provided at some midpoint of the fuel pipe 20. The control valve 22 isopened and closed and opening-controlled by the ECU 9. The fuel additionnozzle 19 is mounted such that fuel is injected toward an exhaustcollector pipe 15. In this embodiment, the fuel pump 12, the fueladdition nozzle 19, the fuel pipe 20, the fuel passage 21, and thecontrol valve 22 constitute the reducing agent adding device.

[0049] In the midway of the EGR pipe 23, there are provided an EGRcooler 24 and an EGR valve 25. The EGR valve 25 is opening-controlled bythe ECU 9 according to the operating condition of the engine 1 tocontrol the exhaust reflux amount. The EGR pipe 23, the EGR cooler 24,and the EGR valve 25 constitute an exhaust gas recycle device (EGR).

[0050] Further, in the exhaust pipe 16, there is provided immediately onthe downstream side of the casing 18 an exhaust gas temperature sensor26 for outputting an output signal corresponding to the temperature ofthe exhaust gas flowing out of the casing 18 to the ECU 9.

[0051] The ECU 9 consists of a digital computer and comprises a ROM(read only memory), a RAM (random access memory), a CPU (centralprocessor unit), an input port, and an output port which are mutuallyconnected by a two-way bus, performing basic controls, such as controlof fuel injection amount of the engine 1.

[0052] For these controls, an input signal from an accelerator openingsensor 28 and an input signal from a crank angle sensor 27 are input tothe input port of the ECU 9. The accelerator opening sensor 28 outputsan output voltage in proportion to the accelerator opening to the ECU 9,and the ECU 9 calculates the engine load on the basis of the outputsignal of the accelerator opening sensor 28. Each time the crank shaftrotates by a fixed angle, the crank angle sensor 27 outputs an outputpulse to the ECU 9, which calculates the engine RPM on the basis of thisoutput pulse. The engine operating condition is judged by the engineload and the engine RPM, and the ECU 9 calculates the fuel injectionamount according to the engine operating condition with reference to aninjection amount map (not shown), calculating the valve opening periodof the fuel injection valve 10 corresponding to the calculated fuelinjection amount to thereby control the operation of the fuel injectionvalve 10.

[0053] Next, the occlusion reduction type NOx catalyst (hereinaftersometimes referred to as NOx catalyst) accommodated in the casing 18will be described.

[0054] The occlusion reduction type NOx catalyst uses, for example,alumina (Al₂O₃), as the support, on which are supported at least oneselected from the group consisting of: an alkali metal, such aspotassium K, sodium Na, lithium Li, or cesium Cs, an alkaline earthmetal, such as barium Ba, or calcium Ca, and a rare earth metal, such aslanthanum La or yttrium Y, and a precious metal, such as platinum Pt.

[0055] This NOx catalyst effects absorption and release operations ofNOx in which it absorbs NOx when the air fuel ratio of the inflowexhaust (hereinafter referred to as exhaust air fuel ratio) is leanerthan the stoichiometric air fuel ratio, and releases the absorbed NOx inthe form of NO₂ or NO when the exhaust air fuel ratio becomes equal toor richer than the stoichiometric air fuel ratio to cause a reduction inthe oxygen concentration of the inflow exhaust gas. Then, NOx (NO₂ orNO) released from the NOx catalyst immediately reacts with the unburnedHC and CO in the exhaust gas to be reduced to N₂. Thus, by appropriatelycontrolling the exhaust air fuel ratio, it is possible to clean the HC,CO, and NOx in the exhaust gas.

[0056] Here, the term exhaust air fuel ratio means the ratio of thetotal amount of air supplied to the exhaust passage on the upstream sideof the NOx catalyst, the engine combustion chamber, the intake passage,etc. to the total amount of fuel (hydrocarbon). Thus, when no fuel,reducing agent, or air is supplied to the exhaust passage on theupstream side of the NOx catalyst, the exhaust air fuel ratio coincideswith the air fuel ratio of the mixture supplied to the engine combustionchamber.

[0057] Note that, in the case of a diesel engine, combustion is effectedin a range in which the air fuel ratio is much leaner than in thestoichiometric range (A/F=14 to 15), so that, in the normal engineoperating condition, the air fuel ratio of the exhaust gas flowing intothe NOx catalyst is very lean, and the NOx in the exhaust gas isabsorbed by the NOx catalyst, the amount of NOx released from the NOxcatalyst being very small.

[0058] Further, in the case of a gasoline engine, the air fuel ratio ofthe exhaust gas is made stoichiometric or rich by making the air fuelratio of the mixture supplied to the combustion chamber stoichiometricor rich, and the oxygen concentration of the exhaust gas is lowered,enabling the NOx absorbed by the NOx catalyst to be released. In thecase of a diesel engine, however, this method cannot be adopted since itinvolves problems, for example, of generation of soot at the time ofcombustion when the air fuel ratio of the mixture supplied to thecombustion chamber is made stoichiometric or rich.

[0059] Thus, in a diesel engine, it is necessary to supply reducingagent to the exhaust gas with a predetermined timing to lower the oxygenconcentration of the exhaust gas before the NOx absorption capacity ofthe NOx catalyst has attained saturation, discharging and reducing theNOx absorbed by the NOx catalyst. Generally speaking, the reducing agentcan may consist of gas oil, which is a fuel for a diesel engine.

[0060] Thus, in this embodiment, the amount of NOx absorbed by the NOxcatalyst is estimated by the ECU 9 from the history of the operatingcondition of the engine 1, and when the estimated amount of NOx attainsa predetermined value, the control valve 22 is kept open for apredetermined period of time to inject a predetermined amount of fuelinto the exhaust from the fuel addition nozzle 19 to thereby lower theoxygen concentration of the exhaust flowing into the NOx catalyst,discharging the NOx absorbed by the NOx catalyst and reducing it to N₂.

[0061] At this time, since the fuel addition nozzle 19 ejects fueltoward the exhaust collector pipe 15, the added fuel flows smoothly tothe exhaust connector pipe 15. The fuel addition nozzle 19 is mounted tothe exhaust port 13 of the fourth cylinder, whereas the connectingposition of the EGR pipe 23 in the exhaust manifold 14 is close to thefirst cylinder, so that there is little possibility of the fuel addedfrom the fuel addition nozzle 19 being allowed to get into the EGR pipe23.

[0062] As stated above, when the NOx catalyst 17 is arranged in theexhaust passage, the NOx absorbing agent supported thereby absorbs andreleases NOx. It is thought that the NOx absorption/reduction mechanismis as shown in FIG. 2. While the drawing illustrates a case in whichplatinum Pt and barium Ba are supported onto a support, a similarmechanism can be also realized by using other precious metals, alkalimetals, alkaline earth metals, or rare earth metals. The mechanism willnow be schematically described.

[0063] First, when the inflow exhaust gas continues to be in a leanstate, the oxygen concentration of the inflow exhaust gas increases,and, as shown in FIG. 2(A), the oxygen O₂, whose amount has beenincreased, adheres to the surface of the platinum Pt as O₂ ⁻ or O²⁻.This O₂ ⁻ or O²⁻ reacts with the NO in the inflow exhaust gas to produceNO₂ (2NO+O₂→2NO₂). A part of the NO₂ thus produced is absorbed by theNOx absorbing agent to be combined with barium oxide BaO while beingfurther oxidized on the platinum Pt. As a result, it is diffused intothe NOx absorbing agent as nitrate ions NO₃ ⁻, as shown in FIG. 2(A). Inthis way, NOx is absorbed by the NOx absorbing agent.

[0064] While the oxygen concentration of the inflow exhaust gas is high,NO₂ is produced on the surface of the platinum Pt. Until the absorptioncapacity of the NOx absorbing agent attains saturation, NO₂ continues tobe absorbed by the NOx absorbing agent to produce nitrate ions NO₃ ⁻.

[0065] On the contrary, when the oxygen concentration of the inflowexhaust gas is lowered and the production amount of NO₂ is reduced, thereaction is reversed (NO₃ ⁻→NO₂), and the nitrate ions NO₃ ⁻ in the NOxabsorbing agent are released from the NOx absorbing agent as NO₂. Thatis, when the oxygen concentration of the inflow exhaust gas is lowered,NOx is released from the NOx absorbing agent. However, when the degreeof leanness of the inflow exhaust gas is low, the oxygen concentrationof the inflow exhaust gas is lowered.

[0066] On the other hand, when the air fuel ratio of the inflow exhaustgas is enriched, the HC and CO react with the oxygen O₂ ⁻ or O²⁻ on theplatinum Pt to be thereby oxidized. Further, when the air fuel ratio ofthe inflow exhaust gas is rich, the oxygen concentration of the inflowexhaust gas is extremely lowered, so that NO₂ is released from the NOxabsorbing agent. As shown in FIG. 2(B), this NO₂reacts with the unburnedHC and CO to be thereby reduced and cleaned. When NO₂ thus ceases toexist on the surface of the platinum Pt, NO₂is successively releasedfrom the NOx absorbing agent.

[0067] Thus, when the air fuel ratio of the inflow exhaust gas isenriched, NOx is released from the NOx absorbing agent and reduced andcleaned in a short time.

[0068] In this embodiment, a diesel engine is used, so that the exhaustair fuel ratio during normal operation is lean, and the NOx absorbingagent absorbs the NOx in the exhaust. When a reducing agent is suppliedto the exhaust port on the upstream side of the NOx catalyst 17, the airfuel ratio of the exhaust gas passing through this NOx catalyst 17becomes rich, and NOx is released from the NOx absorbing agent andreduced.

[0069] Here, the term “exhaust air fuel ratio” means ratio of the air tothe fuel, the air and fuel being supplied to the exhaust port on theupstream side of the NOx absorbing agent and the engine combustionchamber or the intake passage. Thus, when no air or reducing agent issupplied to the exhaust port, the exhaust air fuel ratio is equal to theoperating air fuel ratio of the engine (combustion air fuel ratio in theengine combustion chamber).

[0070] In the present invention, there is no particular limitationregarding the reducing agent as long as it generates a reductioncomponent, such as hydrocarbon or carbon monoxide, in the exhaust.Examples of the reducing agent that can be used include gases, such ashydrogen and carbon monoxide, liquid or gas hydrocarbons, such aspropane, propylene, and butane, and liquid fuels, such as gasoline, gasoil, and kerosene. In this case, as stated above, gas oil, which is thefuel for the engine 1, is used as the reducing agent in order to avoidcomplexity in storage, supply, etc.

[0071] Generally speaking, the reducing agent supplying conditionsinclude the pressure at which the reducing agent (which is fuel in thisembodiment) is supplied (i.e., the injection pressure), the supplyperiod, and the supply interval, which mean nothing other than the fuelsupply amount. Here, the addition period and the addition interval forthe fuel to be added are controlled according to the operatingcondition, whereby NOx is released and reduced efficiently.

[0072] In the following, a case will be described in which, whensupplying reducing agent at the exhaust port 13 from the fuel additionnozzle 19, the load condition of the engine 1 is judged, and theaddition period and the addition interval for the fuel (gas oil) as thereducing agent are controlled according, in particular, to the exhaustvelocity.

[0073] In this control, the operating condition for the engine 1 isfirst read by the ECU 9. As stated above, the ECU 9 calculates theengine load on the basis of the output signal of the accelerator openingsensor 28, and calculates the engine RPM on the basis of the outputpulse of the crank angle sensor 27. The engine operating condition isjudged by the engine load and the engine RPM, and the ECU 9 calculatesthe fuel injection amount according to the engine operating conditionwith reference to an injection amount map (not shown).

[0074] Subsequently, the amount of NOx absorbed by the NOx catalyst isestimated by the ECU 9 from the history of the operating condition ofthe engine 1. When the estimated amount of NOx has reached apredetermined value, fuel is injected into the exhaust from the fueladdition nozzle 19.

[0075] The ECU 9 calculates the engine load as described above. Further,on the basis of a signal from the exhaust gas temperature sensor 26provided on the downstream side of the NOx catalyst 17, it adopts thisas the bed temperature of the NOx catalyst 17. The bed temperature ofthe NOx catalyst 17 is used when making a judgment as to whetherreducing agent is to be supplied or not. When it is not within apredetermined temperature range, that is, when the NOx catalyst 17 isnot within the activation range, no reducing agent is supplied.

[0076] Further, the ECU 9 determines the addition period P for the fuelas the reducing agent from the operating condition based on the amountof fuel Q injected into the engine 1 and the engine RPM Ne. Thisaddition period P is previously obtained by experiment in the form of amap as shown in FIG. 3(B) as a function of the amount of fuel Q injectedinto the engine 1 and the engine RPM Ne, and is stored in the ROM of theECU 9.

[0077] As shown in FIG. 3(A), regarding the addition period P for thefuel, the less the fuel injection amount Q, the longer the additionperiod P for the fuel as the reducing agent, the addition period Pdecreasing as the fuel injection amount Q increases. When the fuelinjection amount Q is small, the addition period P is substantiallyconstant even if the engine RPM Ne is raised. However, as the fuelinjection amount Q increases, the addition period P becomes stillshorter as a result of an increase in the engine RPM Ne.

[0078] On the other hand, as shown in FIG. 4 (A), an addition interval Ifor the fuel is determined such that the less the amount of fuel Qinjected to the engine 1, the longer the addition interval I for thefuel, the addition interval I decreasing as the fuel injection amount Qincreases.

[0079] Further, the addition interval I for the reducing agent ispreviously stored in the ROM of the ECU 9 in the form of a map as shownin FIG. 4(B) as a function of the engine RPM Ne and the fuel injectionamount Q. Normally, when the engine RPM Ne is low, the addition intervalI is long, the addition interval I decreasing as the engine RPM Neincreases.

[0080] Note that, when the pressure acting on the reducing agentinjected from the fuel addition nozzle 19 is constant, the additionperiod P and the reducing agent injection amount are in a fixedrelationship. Thus, by controlling the length of the addition period Pfor the reducing agent as described above, the injection amount of thereducing agent is increased or decreased according to the operatingcondition of the engine 1.

[0081] This reducing agent addition control is executed in accordancewith the reducing agent addition routine shown in FIG. 5. Thisprocessing routine is previously stored in the ROM of the ECU 9, and isrepeatedly executed by the CPU.

[0082] First, in step 100, a judgment is made as to whether NOx is to bereduced and released. When it is determined that reduction and releasingof NOx is not to be executed on the basis of the NOx occlusion amount atthe NOx catalyst 17 estimated from the operation history, the engineoperating condition, the catalyst bed temperature, etc., this routine istemporarily terminated.

[0083] On the other hand, if it is determined that NOx is to be reducedand released on the basis of the operation history, etc., the procedureadvances to step 101.

[0084] In step 101, the ECU 9 reads the engine load and the operatingconditions of the engine 1, such as the engine RPM.

[0085] Then, in step 102, the ECU 9 calculates the addition period andthe addition interval for the fuel as the reducing agent on the basis ofthe operating conditions of the engine 1 read in step 101.

[0086] In step 103, the addition of the fuel is executed on the basis ofthe calculated addition period and addition interval for the fuel.

[0087] While in the above-described embodiment the present invention isapplied to a diesel engine by way of an example, the present inventionis also applicable to a gasoline engine capable of lean burn.

INDUSTRIAL APPLICABILITY

[0088] The exhaust emission control device for an internal combustionengine of the present invention is capable of efficiently cleaning NOxin the exhaust gas discharged from a lean burn engine, such as dieselengine. Thus, it can be widely used, in particular, in vehicle-mountedlean burn engines.

1. An exhaust emission control device for an internal combustion engine,characterized by comprising: an NOx catalyst provided in an exhaustpassage of an internal combustion engine capable of lean burn andadapted to release absorbed NOx by a reducing agent for reduction; areducing agent supplying means provided in the exhaust passage on theupstream side of the NOx catalyst; a load detecting means for detectingthe load of the internal combustion engine; and a reducing agentaddition control means for controlling reducing agent addition periodand reducing agent addition interval on the basis of the load of theinternal combustion engine.
 2. An exhaust emission control device for aninternal combustion engine according to claim 1, characterized in thatthe device further comprises an addition determining means fordetermining as to whether reducing agent is to be added or not accordingto the operating condition of the vehicle, and that the addition amountof reducing agent and the reducing agent addition interval arecontrolled on the basis of the load detected by the load detecting meanswhen it is determined by the addition determining means that addition ispossible.
 3. An exhaust emission control device for an internalcombustion engine according to claim 1 or 2, characterized in that whenthe load of the internal combustion engine is small, the amount ofreducing agent to be added is increased and the reducing agent additioninterval is elongated as compared to the case in which the load of theinternal combustion engine is large.
 4. An exhaust emission controldevice for an internal combustion, characterized by comprising: an NOxcatalyst provided in an exhaust passage of an internal combustion enginecapable of lean burn and adapted to release absorbed NOx by a reducingagent for reduction; a reducing agent supplying means provided in theexhaust passage on the upstream side of the NOx catalyst; an operatingcondition detecting means for detecting the load and RPM of the internalcombustion engine; and a reducing agent addition control means forcontrolling reducing agent addition period and reducing agent additioninterval on the basis of the detected load and RPM of the internalcombustion engine.
 5. An exhaust emission control device for an internalcombustion engine according to claim 1 or 4, characterized in that theamount of reducing agent added is larger and the reducing agent additioninterval is longer when the load of the internal combustion engine issmall and the RPM thereof is low than when the load of the internalcombustion engine is large and the RPM thereof is high.
 6. An exhaustemission control device for an internal combustion engine according toclaim 1 or 4, wherein said NOx catalyst is an occlusion reduction typeNOx catalyst.
 7. An exhaust emission control device for an internalcombustion engine according to claim 1 or 4, wherein said reducing agentsupplying means consists of a reducing agent injection nozzle forinjecting reducing agent into the exhaust passage on the upstream sideof the NOx catalyst.